Tear gas, gunfire and explosions could be heard during the fight near the Government Headquarters. Both sides appeared to be using weapons. Witnesses say at least three police were wounded.

Elsewhere, security officials say about 100 people were detained during a police operation to take back a protest site at the Ministry of Energy. No clashes were reported.

Since late last year, opposition protesters have occupied government compounds and key intersections in Bangkok, in an attempt to overthrow the government.

Police have said they will not use violence, but insist they will clear at least some the demonstrations, which they say are in violation of a state of emergency. Protest leaders have insisted the protests are peaceful and do not contain weapons.

At least 10 people have been killed since November in a series of small-scale clashes and attacks on demonstrators. It is Thailand's worst political violence since 2010.

The protesters are trying to force Prime Minister Yingluck Shinawatra to resign. They say her government is corrupt and controlled by her billionaire brother, the exiled ex-prime minister Thaksin Shinawatra.

Ms. Yingluck tried to resolve the conflict with early elections this month. But the opposition boycotted the vote, and disrupted it in several areas, preventing a definitive result until more polls can be held.